For those of You who realy know: Is the output transistors on the 9.2 isolated from the heatsink, or have the just made this arrangement for attaching the heatsink to make it possible to mount the transistors directly to the heatsink, no mica between?

Long long time ago I had to design a platform of a navy divers vessel, for the scuba guys to embark/disembark from with their gear and/or inflatables & RIBs.
As a 2nd year engineering student, I fiercely started setting up all kinds of differential equations, for an accurate calculation to the last digit.
After half a day, the fulltime construction design guy (with a entry level degree) handed me a book with approximation formulas.

Told me to multiply that by a factor 2 or 3, as the marines wouldn't mount the platform if it looked too shabby.
And the exact size material wouldn't be in store anyway, he said, ordering it through the official government channel would take too long and be more costly. (and that was a multi-million conversion project, me also handled the cost estimate)

Commercial audio appears as one of rare few industries where undersizing or making the mark by a hair's breadth is Not unusual.

Unless you need a PA amplifier an option is to stick with what you
have at hand , 625VA/channel is good enough for 450W RMS/channel
on a permanent output power , 500W RMS/channel can be reached
by a musical program without being as power consuming seen from
the power supply.

I strongly disagree with You. Or do I really?

Try look at the simplest of figures. If I want a continious 250W in 8 Ohms, I need a continious currentflow from the transformer of 5,59A, Wich by a fraction is excactly what the 625VA transformers I have is capable of delivering.
Yes, I will probable find the 450W RMS somewhere in an way, but they tell me nothing of what I can expect

I have no particular interrest in making any Watt-War here, but I stick to the FTC-norm when calculate and meassure watts.
Funfact:
And by using one of SONYs ways of determining output power, I probably a couple of kilowatts too.

But still I stick to my thumb-rule, saying approx 3 times the estimated continious poweroutput. 625 is a bit too small, 800 would have been perfect.

I guess I will get my 258W as the maximum continious power.
All in all I guess I will be happy with that.
On the other hand, I am not quite so sure my wife will be.
2,58W would be hers preferance. Anybody heard similar?

Try look at the simplest of figures. If I want a continious 250W in 8 Ohms, I need a continious currentflow from the transformer of 5,59A, Wich by a fraction is excactly what the 625VA transformers I have is capable of delivering.

The 450W figure wasnt pulled exactly from thin air...

The 5.59A are drained from each rail ALTERNATIVELY , so the
average current from each rail is half this value at 250W/8R ,
in fact a single 625VA xformer is enough for two channels
granted the amp is not used at full power for extensive
timings.

I am afraid You have the maths some wrong here.
To produce the 44V AC needed to produce 250W one need the full power from both rails.
This means alternatively 5,59A from each rail. And there is not room for too much more.

A full curve of 44VAC measures 125V peak to peak.

Actually one already at this point can expect some distortion as the peak level, 62,5V ends up in a peak current of nearly 8 Amps. Luckily there is some capasitors present to aid this to happen.

Actualy the maths is right providing we are talking of a single channel...

The upper side of the push pull will suplly the 5.6A half of the time ,
during positive going of the signal while the bottom side will conduct
only on negative cycles of the signal , hence the power supply rails
will be sollicited alternatively, the average current drained by the amp
on each rail will be 2,3A.

On a class AB/B biaised push pull each side conduct only 50% of the time ,
the non active side conduction being negligible.

According to profusions application note: The power supply should be capable of supplying the module with D.C. equivalent to 150% of the RMS output power. The mains transformer should be sized according to expected usage. Hi-fi amplifiers use a transformer rated at approx 1.5 x the
total output power (in VA). For a stage or P.A. amplifier the factor should be 2 - 2.5 times the output power. Oversize power transformers will
also enhance the sound quality.

scrollin through this i noticed this yamaha comment and it reminded me of young apprentice who said he heard nothing better...never heard myself but now a little interested...still using my bk st 140 and pro 5 but always on look out for something new old and cheep